This invention relates to a method of forming color images by an electrophotographic copying process wherein a photosensitve member is exposed to three light beams each containing image information of one of three primary colors or yellow, magenta and cyan obtained by color separating a colored document.
In one method known in the art of recording color picture images by using yellow, magenta and cyan image signals obtained by color separating a colored document in color by a peripheral color hard copy producing apparatus of a color facsimile system, computer, etc., a photosensitive member is scanned by three laser beams obtained by splitting a laser beam and modulating by color image information signals of the document and recording is performed by an electrophotographic process. In apparatus of the prior art used to carry this method into practice, a photosensitive member is exposed to optical images of three primary colors or yellow, magenta and cyan of the color document to form three electrostatic latent images either in three separate zones of the photosensitive member, by projecting the three color optical images simultaneously or by projecting one color optical image after another while the photosensitive member is rotated in timed relation to the projection of the color optical images. The three electrostatic latent images thus formed are developed by using toners of yellow, magenta and cyan to produce three toner images of different colors which are printed by transfer-printing on a transfer-printing sheet would on a transfer-printing drum in superposed relation in the same position by rotating the transfer-printing drum three times. Alternatively, a laser beam is split into three laser beam components which are each projected against one of three photosensitive drums to form an electrostatic latent image on the surface of each photosensitive drum, and the electrostatic latent images formed on the three separate photosensitive drums are developed by yellow, magenta and cyan toners, respectively, to form toner images of different colors which are printed by transfer-printing in the same position in superposed relation on a transfer-printing sheet which is fed to a transfer-printing station by being successively brought into contact with the three photosensitive drums, to thereby record a color image of the document.
As noted hereinabove, it has hitherto been usual practice in the prior art to print toner images of three different colors by transfer-printing in the same position of a transfer-printing sheet by superposing one toner image of one color after another. This process has suffered various disadvantages. One of them is that variations may be porduced in the positions of toner images of different colors on the transfer-printing sheet. Another disadvantage is that when the electrostatic latent images of different colors are formed in separte zones of the single photosenstiive drum, the photosensitive drum would have to have a large diameter. When three photosensitive drums are used each for forming one of the three electrostatic latent images of different colors, it would be necessary to use three sets of not only the photosensitive drum but also other equipment necessary for performing the electrophotographic copying process. The space required for installing the apparatus for performing the operation would be great and the equipment used would also become large in size. When the single photosensitive drum is rotated three times for recording a color image, the operation of performing recording would take a long period of time, although the space required to install the apparatus to perform the operation would not be large. To project three laser beam components obtained by splitting a laser beam against the photosensitive member in three separate zones in timed relation would require the provision of a plurality of mirrors each for one of the paths of the three laser beam components to separate the zones for projecting the laser beam components or the provision of a shift register to an output circuit of the each of solid state image sensors. This would render the apparatus complex in construction and require adjustment to be effected with a high degree of precision, resulting in a rise in cost.
As noted hereinabove, the electrophotographic color image recording system of the prior art has suffered the disadvantages that a large number of process steps have to be followed in effecting positioning of the images each time an electrostatic latent image of one primary color is developed into a color toner image and the toner images of different colors are superposed one over another to record a color image by transfer-printing, and that consequently the apparatus for carrying such method into practice becomes complex in construction and large in size. These disadvantages have been largely responsible for the fact that the system has not become popular against expectation.